WDM system that uses nonlinear temporal gratings

ABSTRACT

A method of generating a signal including: generating a first sequence of coherent optical pulses which are sufficiently close in spacing so as to overlap and interfere upon traveling a predetermined length down an optical fiber to form a first interference pattern with a first central lobe having a characteristic wavelength, wherein the energy of the pulses of the first sequence of pulses is within the non-linear regime of the optical fiber; manipulating the first sequence of optical pulses; in a similar manner generating a second sequence of coherent optical pulses; manipulating the second sequence of optical pulses; and introducing both the first and second manipulated sequences of pulses into the optical fiber, wherein the manipulating of the pulses of the first and the second sequence of pulses shifts the characteristic wavelengths of the first and second central lobes, respectively, to first and second transmission signal wavelengths.

This application is a continuation of U.S. Ser. No. 09/722,080, entitled“Nonlinear Temporal Grating As A New Optical Solitary Wave” filed Nov.22, 2000 and which claims the benefit of U.S. Provisional ApplicationNo. 60/222,708, filed Aug. 3, 2000 and of U.S. Provisional ApplicationNo. 60/244,298, filed Oct. 30, 2000.

This invention was made with government support under Grant No.F19628-95-C-0002 awarded by the Air Force. The government has certainrights in the invention.

TECHNICAL FIELD

This invention relates generally to transmitting optical signals inoptical fibers, and more particularly, to reducing pulse broadening in anonlinear operating region of optical fiber transmission and ultra-fastoptical switching.

BACKGROUND

FIG. 1 shows that an initial optical pulse 2 becomes a broader pulse 3after traveling through an optical fiber 4. One source of broadening ofpulse 2 results from dispersion. One cause of dispersion is a variationin a fiber's refractive index with wavelength. The fiber's refractiveindex is defined as the ratio of speed of light in vacuum to speed oflight in the fiber. The refractive index variations make longer andshorter wavelength components of pulse 2 travel at different speeds inoptical fiber 4. After traveling through a certain length of opticalfiber 4, the speed variations produce broader pulse 3. Another cause ofdispersion is waveguide dispersion, which is induced by the geometricconfiguration of fiber 4.

Pulse broadening can affect the quality of digital data transmission inoptical fiber 4. Digital data is transmitted as a series of opticalpulses. Each temporal interval for a pulse may represent one binary bit.For example, a data format called On-Off Keying (OOK) indicates thebinary states “1” and “0” corresponding to the presence and absence of apulse, respectively. As pulses broaden and overlap, a receiver may notbe able to determine whether a pulse is present in a particular timeinterval or whether a detected optical signal is the tail of a previousor subsequent pulse. Inserting an amplifier 5 into optical fiber 4 canhelp to reduce receiver errors due to propagation weakening of pulseintensities. But, amplifier 5 does not help to reduce receiver errorscaused by the dispersion-generated pulse broadening and overlap.

Present optical fiber communications typically use optical pulses havingwavelengths of about 1.5 microns, because erbium-doped fibers canprovide quality optical amplification at 1.5 microns. Unfortunately,many older optical fibers produce significant chromatic dispersion inoptical signals at 1.5 microns. This chromatic dispersion producessignificant pulse broadening, which limits transmission wavelengths anddistances in contemporary optical networks.

The refractive index of fiber 4 also varies with the magnitude ofelectric field ∈ of pulse 2. For symmetric molecules, such as silicaglasses of which most optical fibers are made, the first-order ∈dependent term in the refractive index vanishes. The higher-order termsof ∈, ∈², in particular, in the refractive index produce most of thenonlinear effects in optical fibers. When the intensity in pulse 2 islow, the higher-order terms of ∈ in the refractive index only havenegligible effects, and therefore pulse 2 is in a linear operatingregion of fiber 4. When the intensity of pulse 2 is sufficiently high,the higher-order terms of ∈ become non-negligible and cause pulse 2 toenter a nonlinear region of operation of fiber 4.

A notable manifestation of the nonlinear operation of fibers isself-phase modulation (SPM). SPM generally causes a pulse to broaden inspectrum while the pulse is propagating in the nonlinear operationregion of a fiber. However, the effects of spectral broadening caused bySPM may counterbalance the effects of chromatic dispersion with theresult that the pulse retains its shape.

The chromatic dispersion is characterized by a second order chromaticdispersion parameter β₂, which is a function of the pulse's wavelengthand derivatives of the fiber's refractive index with respect to thewavelength. If β₂ is negative, the pulse is said to be propagating in ananomalous dispersion regime of the fiber. In the anomalous dispersionregime, the SPM causes the leading edge of the pulse to travel slowerthan its trailing edge, thus effectively compressing the pulse andbalancing out the pulse broadening induced by the second order chromaticdispersion.

A pulse propagating in the fiber with balanced SPM and chromaticdispersion is a form of solitary wave called a soliton. Ideally, asoliton may travel a long distance while retaining its shape andspectrum. However, a soliton is susceptible to amplitude fluctuations,which may be caused by, for example, the amplifiers that are requiredalong the fiber. The amplitude fluctuations generate frequency shifts,which in turn cause Gordon-Haus time jitters due to differentfrequencies traveling at different velocities. The frequency shifts andGordon-Haus time jitters are detrimental to a data transmission system.In a wave-length division multiplexing (WDM) system, frequency shiftsproduce undesired emissions outside of the allotted frequency bandassigned to each channel, and the undesired emissions may interfere withother channels or other systems; while time jitters create problems ofdata clock recovery at a receiver or regenerator site, because data bitsrepresented by the optical pulses may not be synchronized due to thetiming uncertainties.

Time jitters can be reduced by inserting sliding filters instrategically chosen locations along the fiber span. Another method toreduce time jitters is a dispersion-managed soliton technique that usesdispersion compensating fibers, which have dispersion characteristicstuned to compensate for the time jitters along the fiber span. Theoverall average dispersion characteristicis, on the other hand, isdesigned to counterbalance the SPM.

Even with dispersion management, any soliton, when traveling far enoughinto a fiber, surrenders to an effect called third order dispersion(TOD). TOD causes the soliton to spread unsymmetrically in the temporaldomain into a widened, non-symmetrical pulse. FIGS. 2 illustrates theTOD effects on a soliton. FIG. 2 shows a soliton pulse after beingunsymmetrically spread by TOD.

Some implementations allow a soliton to travel over long distance withoptical regenerators. The design of regenerators, for example, optical2R (re-shape and re-time) or 3R (re-shape, re-time and re-amplify),involves complicated issues such as polarization sensitivities, cost andcomplexities.

SUMMARY

In general, in one aspect, the invention is a method of generating asignal pulse in an optical fiber characterized by dispersion and arefraction index that has a nonlinear regime of operation. The methodinvolves generating a sequence of coherent optical pulses each of whichhas an associated energy; and introducing the sequence of pulses intothe optical fiber, wherein the pulses in the sequence of pulses aresufficiently close in spacing so that after traveling a predeterminedlength down the optical fiber, the pulses of the sequence of pulsesoverlap and interfere to form an interference pattern. The associatedenergy of at least one of the pulses of the sequence of pulses is withinthe nonlinear regime of the optical fiber.

In general, in another aspect, the invention is a method of generating asignal pulse in an optical fiber that involves generating a sequence ofcoherent optical pulses each of which has an associated energy; andintroducing the sequence of pulses into the optical fiber, wherein thepulses in the sequence of pulses are sufficiently close in spacing sothat after traveling a predetermined length down the optical fiber, thepulses of the sequence of pulses overlap and interfere to form aninterference pattern having a central lobe and multiple side lobes. Theinterference pattern is characterized by a contrast ratio, and theassociated energy of each pulse of the sequence of pulses issufficiently high relative to characteristics of the optical fiber so asto cause the contrast ratio of the interference pattern to increase asthe interference pattern propagates further along the optical fiber.

In general, in still another aspect, the invention is a method ofgenerating a signal pulse in an optical fiber that involves generating asequence of coherent optical pulses each of which has an associatedenergy; and introducing the sequence of pulses into the optical fiber,wherein the pulses in the sequence of pulses are sufficiently close inspacing so that after traveling a predetermined length down the opticalfiber, the pulses of the sequence of pulses overlap and interfere toform an interference pattern having a central lobe and multiple sidelobes. The associated energy of each pulse of the sequence of pulses issufficiently high relative to characteristics of the optical fiber so asto cause energy from the side lobes to transfer into the central lobe asthe interference pattern propagates further along the optical fiber.

Preferred embodiments include one or more of the following features.Each of the pulses of the sequence of pulses has energy that is withinthe nonlinear regime of the optical fiber. The sequence of pulses mayinclude only two pulses or it may include more than two pulses. Thegenerating of a sequence of coherent optical pulses involves supplying acontinuous wave laser beam; and chopping the continuous wave laser beamto produce the sequence of optical pulses. Alternatively, the method ofgenerating the sequence of coherent optical pulses involves supplying asingle coherent optical pulse; and producing the sequence of opticalpulses from the single optical pulse.

In general in still another aspect, the invention is a system forgenerating a signal pulse in an optical fiber characterized bydispersion and a refraction index that has a nonlinear regime ofoperation. The system includes a source of coherent laser energy; and atransmitter for coupling to the optical fiber and which duringoperation, receives the laser energy from the source and outputs asequence of coherent optical pulses. The transmitter is configured togenerate the pulses in the sequence of pulses with sufficiently closespacing so that after traveling a predetermined length down the opticalfiber, the pulses of the sequence of pulses overlap and interfere toform an interference pattern. The transmitter is also configured togenerate at least one pulse of the sequence of pulses to have an energythat is within the nonlinear regime of the optical fiber.

Preferred embodiments include one or more of the following features. Thetransmitter is configured to generate each of the pulses of the sequenceof pulses to have an energy that is within the nonlinear regime of theoptical fiber. The sequence of pulses may include only two pulses or itmay include more than two pulses. If the source of coherent laser energyprovides a continuous wave optical beam, the transmitter might theninclude an optical shutter that during operation chops the continuousoptical beam to produce the sequence of optical pulses. Alternatively,it the source of coherent light supplies a single coherent opticalpulse, then the transmitter might include a splitter that receives thesingle pulse, a plurality of optical paths connected to an output of thesplitter, each of the plurality of optical paths characterized by adifferent delay, and a combiner receiving each of the plurality ofoptical paths and during operation outputting the sequence of opticalpulses.

In general, in yet another aspect, the invention is a method ofgenerating a signal for transmission down an optical fiber characterizedby dispersion and a non-linear regime of operation. The method includes:generating a first sequence of coherent optical pulses each of which hasan associated energy, wherein the pulses in the first sequence ofoptical pulses are sufficiently close in spacing so that upon travelinga predetermined length down the optical fiber, the pulses of the firstsequence of pulses will overlap and interfere to form a firstinterference pattern with a first central lobe having a characteristicwavelength, and wherein the associated energy of at least one of thepulses of the first sequence of pulses is within the non-linear regimeof the optical fiber; and manipulating the pulses of the first sequenceof optical pulses to generate a first manipulated sequence of pulses. Itfurther includes generating a second sequence of coherent optical pulseseach of which has an associated energy, wherein the pulses in the secondsequence of optical pulses are sufficiently close in spacing so thatupon traveling a predetermined length down the optical fiber, the pulsesof the second sequence of pulses will overlap and interfere to form asecond interference pattern with a second central lobe having acharacteristic wavelength, and wherein the associated energy of at leastone of the pulses of the second sequence of pulses is within thenon-linear regime of the optical fiber; and manipulating the pulses ofthe second sequence of optical pulses to generate a second manipulatedsequence of pulses. The method also includes introducing both the firstmanipulated sequence of pulses and the second manipulated sequence ofpulses into the optical fiber, wherein the manipulating of the pulses ofthe first and the second sequence of pulses shifts the characteristicwavelengths of the first and second central lobes, respectively, tofirst and second transmission signal wavelengths.

In general, in still yet another aspect, the invention features anapparatus for sending optical signals down an optical fibercharacterized by dispersion and a non-linear regime of operation. Theapparatus includes: a first optical signal source which during operationgenerates a first sequence of coherent optical pulses each of which hasan associated energy, wherein the pulses in the first sequence ofoptical pulses are sufficiently close in spacing so that upon travelinga predetermined length down the optical fiber, the pulses of the firstsequence of pulses will overlap and interfere to form a firstinterference pattern with a first central lobe having a characteristicwavelength, and wherein the associated energy of at least one of thepulses of the first sequence of pulses is within the non-linear regimeof the optical fiber; a frequency shifter which during operationmanipulates the pulses of the first sequence of optical pulses togenerate a first manipulated sequence of pulses; a second optical signalsource which during operation generates a second sequence of coherentoptical pulses each of which has an associated energy, wherein thepulses in the second sequence of optical pulses are sufficiently closein spacing so that upon traveling a predetermined length down theoptical fiber, the pulses of the second sequence of pulses will overlapand interfere to form a second interference pattern with a secondcentral lobe having a characteristic wavelength, and wherein theassociated energy of at least one of the pulses of the second sequenceof pulses is within the non-linear regime of the optical fiber; a secondfrequency shifter which during operation manipulates the pulses of thesecond sequence of optical pulses to generate a second manipulatedsequence of pulses; and a wavelength division multiplexer system whichduring operation receives both the first manipulated sequence of pulsesand the second manipulated sequence of pulses and introduces them intothe optical fiber, wherein the first and second frequency shifters shiftthe characteristic wavelengths of the first and second central lobes,respectively, to first and second transmission signal wavelengths.

Embodiments may have one or more of the following advantages. A newoptical solitary wave, the hyper-soliton, is discovered for transmittingin the non-linear operating region of an optical fiber. Thehyper-soliton does not spread as it travels down the fiber, and carriesdigital signals over a broad frequency range. Further aspects, featuresand advantages will become apparent by the following.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates pulse broadening in a prior art optical fiber;

FIG. 2 shows a soliton pulse after being spread by TOD;

FIG. 3 shows a system, which uses interference to reduce pulsebroadening in a linear operating region of an optical fiber;

FIG. 4 illustrates an interference pattern produced from the system ofFIG. 3;

FIG. 5 illustrates diffraction broadening caused by a wide slit;

FIG. 6 illustrates multi-slit interference;

FIG. 7 shows a system, which uses interference to reduce pulsebroadening in a nonlinear operating region of an optical fiber;

FIG. 8 illustrates an interference pattern produced from the system ofFIG. 7;

FIG. 9 shows an optical transmitter for use in the system of FIG. 7;

FIGS. 10A-C illustrate Mach-Zehnder interferometers;

FIG. 10D illustrates how the variable delay component of theinterferometer adjusts the delay between pulses;

FIGS. 11A-D illustrate how the interferometer adjusts the pulseamplitude;

FIG. 12 shows another optical transmitter for use in the system of FIG.7;

FIG. 13 shows another optical transmitter containing a pulse splitterfor use in the system of FIG. 7;

FIG. 14 shows a 1×5 optical beam splitter for use in the transmitter ofFIG. 12;

FIGS. 15A and 15B illustrate temporal shiftsproduced by the systems ofFIG. 3 and FIG. 7, respectively;

FIG. 16 shows a wavelength division optical transmission system; and

FIG. 17 shows a wavelength division optical transmission receiver.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

The present invention is based, in part, on a multi-pulse techniquewhich we developed to reduce the pulse broadening effects of dispersionon a transmitted pulse. Our multi-pulse technique involves generatingfor each pulse to be transmitted a series of closely spaced coherentpulses and then sending that series of closely spaced pulses down thefiber. According to the multi-pulse technique, the individual pulseswithin the series of pulses are selected to have amplitudes that arewithin the linear operating region of the fiber and they are spacedclosely enough to interfere with each other as they broaden. Thetechnique is described in detail in U.S. patent application Ser. No.09/282,880, entitled “Quasi-Dispersionless Optical Fiber Transmission,Dispersion Compensation and Optical Clock,” incorporated herein byreference. To lay the groundwork for discussing the present invention,we will first present an overview of the multi-pulse technique.

FIG. 3 schematically illustrates a system 50 which implements themulti-pulse technique. It includes an optical transmitter 54 connectedto a receiver 65 at a distant location 15 via an optical fiber 52.Transmitter 54 generates a sequence of coherent, closely spaced pulses56-59 and sends them into optical fiber 52. Each of the pulses 56-59 hasa nonzero time delay with respect to the preceding pulse in the sequenceof pulses. As each of the pulses 56-59 moves down the fiber 52, it willbroaden due to dispersion in the fiber. If the original pulses in thesequence are spaced closely enough, as they broaden, they will overlapand will interfere with each other. For example, pulses 57 and 58broaden to become overlapping pulses 61 and 62 after a certainpropagation distance.

FIG. 4 shows an example of the resulting interference pattern 80 that isproduced by the overlapping coherent pulses. Note that the interferencepattern typically has a narrow central lobe 69 and multiple side lobes70 and 71 of lower amplitude. The peaks of central lobe 69 and sidelobes 70 and 71 fall under an envelope 18, which matches the shape of abroadened pulse that would result from sending one of the pulses 56-59down fiber 52. Effectively, the sequence of coherent pulses 56-59 formsa single pulse (i.e., central lobe 69) that is able to broaden lessrapidly when propagating through fiber 52. The reason for the slowerrate of spreading is because the pulse sequence acts as a spectralfilter on the resulting single pulse. However, distortions introduced bythe spectral filter can have an irreversible effect on the spectrum ofthe transmitted pulse, which is inversely proportional to its temporalduration. Therefore, with spectral filtering, it may be not possible torecover the temporal duration of the transmitted pulse, which is adrawback that can reduce the potential data rate of the transmissionsystem.

Referring to FIGS. 3 and 4, central lobe 69 is extracted or detected bya nonlinear device at receiver 65, e.g., an intensity discriminator. Thenonlinear device, using a threshold that is higher than the side lobesbut lower than the central lobe, detects the existence of the highercentral lobe and ignores the lower amplitude side lobes. It should beclear that effective operation of this system depends on there beingsufficient contrast between the central lobe and the other side lobesand any undesired noise that might be present. Also note that if only asingle pulse had been transmitted down the fiber, the pulse broadeningeffect of dispersion would probably have produced a low amplitude, widepulse by the time it reached the receiver. The wider, lower pulse wouldhave been difficult if not impossible to detect over the noise.

We also note that when the sequence of pulses is in the linear region ofthe fiber, the contrast ratio of the central lobe to the side lobesremain the same as the pulse sequence travels down the fiber (as shownin FIG. 4), even though temporal waveform of the pulse sequence is beingstretched due to dispersion. We make this observation at this pointbecause as will become apparent later when the signal is in thenon-linear region of the fiber, the contrast ratio of the pulse sequenceno longer remains the same when it propagates down the fiber.

It is useful to observe that interference pattern 80 is similar to theintensity pattern that is produced by coherent plane wave light afterpassing through a multiple aperture grating and the envelope 18 issimilar to the intensity pattern produced by a coherent light beam afterpassing through a single slit aperture. The diffraction effect of asingle aperture is illustrated in FIG. 5 which shows an intensitypattern 20, which a coherent light beam 22 makes on a screen 24 locatedin the far field behind a wide slit 26. If the slit 26 is not too wide,diffraction broadens the intensity pattern 20 to beyond the width of theslit 26.

The interference pattern produced by a multiple aperture grating isillustrated in FIG. 6 which shows an intensity pattern 28, which thecoherent light beam 22 produces on the screen 24 when located behindmultiple narrow slits 30-34. The intensity pattern 28 has lobes 36-38and minima 42-44 due to interference between light waves from thedifferent slits 30-34. The interfering light impinging on screen 24 ischaracterized by a central lobe 36 and multiple side lobes 37, 38.Central lobe 36, as in the case of the sequence of coherent pulsestraveling down a fiber, is much narrower than the diffraction-widenedpattern 20 from the wide slit 26 of FIG. 5. And the envelope 40 of thepeaks of the multiple lobes of the interference pattern 28 matches thepattern 20 from the wide slit 26 of FIG. 5.

In other words, the sequence of closely spaced coherent pulses thatproduce the multi-lobe interference pattern in the fiber might be viewedas having associated therewith a temporal grating (TG) much like thespatial grating that also produces a multi-lobe interference pattern.

By increasing the energy in pulses 56-59 so that they are in thenonlinear region of the fiber, significant performance enhancements areachieved. In essence, when the system is operated in the nonlinearregion, the central lobe of the resulting interference pattern increasesin height and the side lobes are significantly suppressed in comparisonto the interference pattern that is produced when operating within thelinear region. Continuing the grating analogy mentioned above, thenonlinear mode of operation is like associating with the sequence ofpulses a nonlinear temporal grating (NTG). This results in a new type ofsolitary wave that we call a hyper-soliton and that is fundamentallydifferent from temporal grating pulse. Unlike the central lobe oftemporal grating, the central lobe of the nonlinear temporal grating(i.e., hyper-soliton) would not spread as the pulse travels down thefiber. In that respect, it is similar to a soliton. The transformationfrom temporal grating to hyper soliton (nonlinear temporal grating) isakin to the transformation from return-to-zero (RZ) pulse transmissionto soliton transmission in a fiber optic link. We will refer hereinafterto the sequence of pulses that operate within the nonlinear region ofthe fiber as HS (hypersoliton) pulses.

A system which operates in the nonlinear region is shown in FIG. 7. Likethe previously described system, it includes a transmitter 154 connectedto a receiver 165 over an optical fiber 52. In this case, however,transmitter 154 generates a series of coherent pulses 156-159 that arewithin the nonlinear region of fiber 52. Because operation in thenonlinear region produces an interference pattern with an enhancedcentral lobe, receiver 165 does not require and therefore does notinclude an intensity discriminator to isolate the central pulse of theinterference pattern from the side lobes.

For this mode of operation, the energy in each of the pulses 156-159needs to be above an energy threshold which defines a lower boundary forthe nonlinear operating region of fiber 52. The energy threshold of apulse in a given fiber can be determined by a nonlinear phase shift thatis produced within the pulse. When the energy of a pulse causes thenonlinear phase shift to be π or larger, the energy of the pulse hasexceeded the energy threshold and the pulse has entered the nonlinearregion of the fiber. More specifically, the energy threshold can bederived from an equation that defines the relationship between theenergy E and the non-linear phase shift φ. The equation generallyincludes factors such as fiber and pulse characteristics. The equationis φ=2π n₂ EL/(λτ A), where n₂ is the non-linear Kerr coefficient(n₂=3.2×10⁻²⁰ meter square/watts), E is the pulse energy in watts xsecond, L is the length of the fiber (meter), λ is the wavelength(meter), τ is the pulse width (second), and A is the effective fibercore area (meter square) in the propagating mode field.

As in the case of the lower energy pulses, the HS pulses after travelinga sufficient distance overlap and produce an interference pattern suchas is shown in FIG. 8. That sufficient distance is several times thecharacteristic length which is equal to τ²/β₂, where τ is the pulseduration, and β₂ is the second order chromatic parameter of the system.

The nonlinearity acts to concentrate the energy of the multiple pulsesinto central lobe 67 of the interference-generated pattern 64 and drawsenergy away from the side lobes 66, 68 thereby suppressing the sidelobes and increasing the central lobe, which, in turn, increases thecontrast between central lobe 67 and side lobes 66,68, as previouslynoted. Compared with the interference pattern resulting from linearoperation, the HS interference pattern has a significantly highercontrast ratio. Moreover, the narrow central lobe of the interferencepattern retains its shape as it propagates down the fiber. The HS pulsesnot only balance the SPM and the second-order chromatic dispersion likea soliton, but also remain substantially unaffected by the presence ofTOD and higher-order dispersions. Unlike a soliton that must betransmitted in the anomalous dispersion regime to retain its shape, theHS pulses may be transmitted in either the normal dispersion regime orthe anomalous dispersion regime.

Referring again to FIG. 8, it should be noted that in nonlinear regimethe shape of waveforms in both time and frequency domains evolve in asimilar manner as the pulse sequence travels.

The advantages of transmitting HS pulses are numerous. For example, HSpulses are robust and resistant to polarization mode dispersion (PMD) ofany installed optical fiber link. PMD is a result of randombirefringence in the fiber, and is a major problem in a high-speed longhaul system. Furthermore, HS pulses are immune to frequency shifts andtime jitters, both of which, as explained in the background, aredetrimental to a transmission system. Because HS pulses effectivelyresult from applying a spectral filter that travels with a soliton, thefilter removes any frequency shifts and time jitters that wouldotherwise have been present in the HS pulses.

There are a number of different ways to generate the HS pulses, some ofwhich will now be described. In general, transmitter 154 includes alaser source and one or more programmable or configurable componentsthat produce sequences of pulses with desired spacing and amplitude. Ifthe pulses are to carry encoded digital data, an encoder will also beincluded in transmitter 154. The laser source may be a continuous-wave(CW) laser that generates a monochromatic coherent light beam In thatcase, a programmable optical shutter chops the light from the CW laserto produce a sequence of mutually coherent pulses.

Alternatively, referring to FIG. 9, a pulse laser source 174, e.g. a 10GHz pulse laser, may be used to generate a sequence of equally-spacednarrow pulses that may be spaced more closely than desired. In thatcase, a programmable optical shutter 177 allows some of the pulses topass (178 and 178′) while blocking out other pulses in order to generatea sequence of more widely spaced narrow pulses. The distance between thepulses, e.g., pulses 178 and 178′, should be great enough to avoid onepulse from interfering with a neighboring pulse after dispersionwidening has taken place.

In one embodiment of transmitter 154, there is a bidirectional,multi-stage Mach-Zehnder interferometer 179 which generates a sequenceof closely-spaced mutually coherent pulses from each single pulse 178coming from shutter 177. Additionally, interferometer 179 is alsocapable of adjusting the delays and amplitudes for the sequence ofpulses to optimize performance, e.g. to control the shape of theinterference pulse that the sequence of closely-space pulses produceafter broadening.

Referring to FIG. 10A, a single-stage Mach-Zehnder interferometerincludes two arms, a splitter 161, and a combiner 162. Splitter 161splits an incoming pulse into two equal intensity pulses, each of whichis sent down a corresponding one of the two arms. One arm of theinterferometer has a variable delay element 160A for adjusting the delayon the pulse traveling in that arm. The other arm has no variable delayelement. Thus, the pulse in one arm is delayed relative to the pulse inthe other arm. By varying the delay introduced by the variable delayelement, the delay between the two pulses can be changed therebychanging the relative phases of the two pulses. Thus, variable delayelement 160A may also be referred to as a variable phase shifter.Combiner 161 recombines the two pulses to form a sequence of two pulsesthat are delayed relative to each other.

FIGS. 10B and 10C illustrate a two-stage and a three-stage Mach-Zehnderinterferometer, respectively. The multistage Mach-Zehnderinterferometers are constructed by serially linking multiple singlestages. However, in the embodiments we have shown, all stages prior tothe last one have a fixed delay element instead of the variable delayelement, as described for the single stage Mach-Zehnder interferometerof FIG. 10A and only the last stage includes the variable delay element.Of course, one could also construct multistage Mach-Zehnderinterferometers in which more than the last stage includes a variabledelay element, and the amount of delay introduced by each delay elementmay be different from the ones illustrated in FIG. 10. But for purposesof explaining the operation of them, the embodiments we have chosen toillustrate are easier to understand.

The amount of delay introduced by each delay element is a factor of twogreater than the delay introduced by the previous stage. That is, if ainterferometer has N stages that are numbered from 0 to N−1, the delayin stage N−n is 2^((N−n))T and the variable delay element in the laststage has the longest delay equal 2^((N−n))T plus an adjustment. Itshould be readily apparent that since each stage introducesprogressively longer delay and the last stage introduces the longestdelay that the last stage essentially controls the delay between twogroups of pulses. For example, referring to FIG. 10D which shows atwo-stage Mach-Zehnder interferometer, the variable delay element 160Dcontrols the separation in time between the pulses in the first group(i.e., pulses b1 and b2) and the pulses in the second group (i.e.,pulses b3 and b4). If the adjusted delay is δ=−0.2 t, the delay betweenthe group of two pulses (b1-b2 and b3-b4) is changed from 2t to 2t+δ,i.e., 1.8 t, and therefore the delays between the adjacent pulses of thefour pulse sequence become t, 0.8 t and t, respectively. Similarly, inthe three-stage Mach-Zehnder interferometer of FIG. 10C, the delaybetween the two groups of four pulses (i.e., pulses c1-c4 in the firstgroup and pulses c5-c8 in the second group) can be changed from 4t to4t+δ by adjusting the variable delay component 160C in the last stage.

FIGS. 11A-D illustrate four alternative ways to adjust the amplitudes ofthe generated pulses. The pulses with adjusted amplitudes form anapodized pulse array. FIGS. 11A and 11B show, respectively, asingle-stage and a two-stage interferometer with an additionalattenuator in one arm of one of the stages. The attenuator changes theamplitude of the pulse passing through that arm. FIG. 11C illustrates away of adjusting the amplitude without using an attenuator. If the delayintroduced by variable delay element 160A is T instead of 2T as in FIG.10B, the two pulses that are combined will overlay each other and form apulse with approximately twice the amplitude. FIG. 11D illustrates thatthe amplitude may be adjusted with a splitter 161′ that puts differentweights on the two arms. In general, an X/Y splitter puts a weight of Xin one arm and a weight of Y in the other arm, therefore, a 70/30splitter splits an incoming pulse into two pulses with an amplituderatio of 70 to 30.

With the delay and amplitude adjustability of the interferometer, theperformance of transmitter 154 can be readily fine-tuned for optimalnon-linear transmissions.

Another embodiment of transmitter 155 shown in FIG. 12 contains onlypulse laser 175 and programmable shutter 177. Pulse laser 175 generatesclosely spaced coherent pulses having separations of the desired amountfor producing the multiple pulses in a group which eventually interferesto form the interference pulse. Shutter 177 carves out pulses from hestream of pulses 176 generated by pulse laser 175. For example, shutter177 may allow the first two pulses in every five pulses to pass through.Thus, shutter 177 creates groups of pulses with uniform spacing betweenthe groups. For a pulse laser that produces a sequence of pulses with 20ps pulse duration, the spacing between any adjacent groups of five is100 ps. Thus, transmitter 154 transmits digital data bit at a rate of 10Gb/s, with each bit carried by the two pulses in each group of five.

Another embodiment of transmitter 153 is illustrated in FIG. 13.Transmitter 154 includes a CW laser source 74 that produces amonochromatic coherent light beam 76. Programmable high-speed shutter177 in transmitter 153 chops light beam 76 to generate a sequence ofsource pulses 78. Each source pulse 78 enters a pulse splitter 79, whichproduces a series of N delayed and coherent pulses from the source pulseand sends the series of pulses to optical fiber 52.

Still referring to FIG. 13, pulse splitter 79 uses a 1×N beam splitter88, e.g., a 1×N fiber coupler, to produce N mutually coherent pulsesfrom each source pulse. The 1×N beam splitter 88 has an optical outputalong each of N directions, and each output couples to an opticalwaveguide P₁-P_(N), e.g., optical fibers. Each optical waveguideP₁-P_(N) has an optical length measured to produce one of the temporaldelays of the series of pulses 156-159 of FIG. 7. Optical waveguidesP₁-P_(N) couple to an inverted 1×N beam splitter 82 that recombines thedelayed pulses to produce the series of pulses 156-159 shown in FIG. 7.Pulse splitter 79 may also include optical amplifiers (not shown) eitherin the separate waveguides P₁-P_(N) or at its output.

Referring to FIG. 14, an embodiment of the 1×N optical beam splitter isa planar integrated optical splitter 90, which can function as the 1×Noptical beam splitter 88 (for N=5). Optical splitter 90 has an inputhole 92. Hole 92 diffracts each received source pulse into five mutuallycoherent pulses, which are directed along different directions. Eachmutually coherent pulse is collected by a separate optical waveguide94-99, which carries the pulse to an optical conduit P₁-P₅. Opticalwaveguides P₁-P₅ can be continuations of waveguides 94-99 or opticalfibers of various lengths. Various other embodiments of the beamsplitter are described in U.S. patent application Ser. No. 09/282,880,filed Mar. 31, 1999, and incorporated herein by reference .

By changing the phasing and/or amplitude of the pulses within thesequence of closely-spaced coherent pulses, it is possible to shift thecentral lobe of the interference signal. The central lobe enhancementeffect and the suppression of side lobes persists even when the centrallobe of the temporal signal is shifted to either side of the center ofthe envelope. FIG. 15B shows the interference signal produced by asystem operating in the nonlinear region and in which the phasing of thepulses has been adjusted to shift the central lobe toward the left sideof the envelope.

The leftward shift of the center lobe of pattern 238 implies a change inthe time at which the pulse will arrive at the receiver. In other words,shifting the center lobe is a way of introducing a delay in the pulse.In addition, it is also the case that shifting the center lobe in thetime domain produces a shift towards the same direction in the centerlobe in the frequency domain. In other words, the spectral content ofthe pulse can be altered.

A similar phenomenon occurs in the case of liner operation. However, thepresence of the side lobes in the linear case seriously limits theamount of shift that can be introduced without producing an ambiguoussignal. For example, FIG. 15A shows a shifted pattern 202 generatedthrough linear operation. Notice that as the center lobe shifts to theleft, the side lobe on the right also shifts to the left and grows inamplitude. Soon the side lobe will be of comparable amplitude to theshifted center lobe and it will not be possible to discriminate betweenthe two. In contrast, the HS pulses yield a much larger dynmaic range ascompared to operation in the linear region. That is, the center lobe canbe shifted much farther before the much lower amplitude side lobespresent a problem. Because of the wider dynamic range, a system thattransmits HS pulses is truly a broadband system.

As we noted above, the frequency of the HS pulses can also be steered byadjusting the relative phases (i.e., time delays) between the pulses inthe sequence of closely-spaced coherent pulses. Thus, for example,interferometer 179 of FIG. 9 and pulse splitter 79 of FIG. 14 may eachbe used as frequency shifters to convert the frequency of source pulsesinto a pre-selectable frequency for transmission.

The time and frequency steering features can be used to transmit digitaldata on an optical fiber. For example, the time steering feature may beused for a digital data transmission format called Pulse PositionModulation (PPM). According to the PPM format, in a temporal intervalduring which a digital bit is transmitted, the binary states “1” and “0”are indicated by the presence of a pulse in the first and second half ofthe interval, respectively; or vice versa. The frequency steeringfeature may be used for another data format called Frequency ShiftKeying (FSK). According to the FSK format, the binary states “1” and “0”are indicated by a pulse transmitted in the temporal interval withfrequency f₁, and f₂, respectively.

The frequency steering feature is also highly useful for providingmultiple channels in a multi-access digital network. FIG. 16 shows anembodiment of a nonlinear optical transmission system 252 with atransmitter 253. Each channel of transmitter 253 includes a pulse lasersource (S1-S5), an optical programmable shutter 177, an encoder 280, anda channelizer 250. Encoder 280 encodes the pulses from the output ofshutter 177 according to a pre-defined data format, e.g., PPM or FSK.Channelizer 250 then shifts the frequency of the pulses to apre-determined output frequency for that channel. Both encoder 280 andchannelizer 250 may be implemented by Mach-Zehnder interferometers 179,as previously described.

All of the laser sources (S1-S5) may generate pulses with the samecenter frequency f_(c). Channelizer 250 shifts the frequency of thepulses for each of the input sources (S1-S5) to a distinct outputfrequency. As illustrated in FIG. 16, the frequencies and wavelengthsgenerated at the output of the five transmitters are f1-f5 and λ1-λ5,respectively.

Transmitter 253 further includes a wavelength-division multiplexing(WDM) device 251 to multiplex signals from multiple sources into thesame fiber 52. Fiber 52 carries multiple WDM channels, with each of thechannels characterized by a unique wavelength. Each channel carries adata stream that can be encoded independent of other data streams. Thus,the WDM technique increases transmission capacity without requiringelectronics of higher speed to process each channel.

WDM device 251 and frequency shifters 250 are in general bi-directional.Therefore, the same WDM device 251 and frequency shifters 250 used intransmitter 253 may be used in a receiver 290. Referring to FIG. 17, WDMdevice 251 at receiver 290 directs the multiplexed data streams in fiber52 towards five decoders 254, each of the decoders receiving a datastream characterized by a unique wavelengths. Decoder 254 then decodesreceived data format (e.g., PPM or FSK). Similar to transmitter 253,decoder 254 may be implemented by Mach-Zehnder interferometers 179.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A method of generating a signal for transmissiondown an optical fiber characterized by dispersion and a non-linearregime of operation, said method comprising: generating a first sequenceof coherent optical pulses each of which has an associated energy,wherein the pulses in the first sequence of optical pulses aresufficiently close in spacing so that upon traveling a predeterminedlength down the optical fiber, the pulses of the first sequence ofpulses will overlap and interfere to form a first interference patternwith a first central lobe having a characteristic wavelength, andwherein the associated energy of at least one of the pulses of the firstsequence of pulses is within the non-linear regime of the optical fiber;manipulating the pulses of the first sequence of optical pulses togenerate a first manipulated sequence of pulses; generating a secondsequence of coherent optical pulses each of which has an associatedenergy, wherein the pulses in the second sequence of optical pulses aresufficiently close in spacing so that upon traveling a predeterminedlength down the optical fiber, the pulses of the second sequence ofpulses will overlap and interfere to form a second interference patternwith a second central lobe having a characteristic wavelength, andwherein the associated energy of at least one of the pulses of thesecond sequence of pulses is within the non-linear regime of the opticalfiber; manipulating the pulses of the second sequence of optical pulsesto generate a second manipulated sequence of pulses; and introducingboth the first manipulated sequence of pulses and the second manipulatedsequence of pulses into said optical fiber, wherein the manipulating ofthe pulses of the first and the second sequence of pulses shifts thecharacteristic wavelengths of the first and second central lobes,respectively, to first and second transmission signal wavelengths. 2.The method of claim 1 wherein each of the pulses of said first sequenceof pulses has energy that is within the non-linear regime of the opticalfiber.
 3. The method of claim 2 wherein each of the pulses of saidsecond sequence of pulses has energy that is within the non-linearregime of the optical fiber.
 4. The method of claim 1 wherein the stepof manipulating the pulses of the first sequence of optical pulsescomprises changing at least one of amplitude, phase, and timing of thepulses of the first sequence of pulses relative to each other.
 5. Themethod of claim 4 wherein the step of manipulating the pulses of thesecond sequence of optical pulses comprises changing at least one ofamplitude, phase, and timing of the pulses of the second sequence ofpulses relative to each other.
 6. The method of claim 1 wherein thefirst sequence of pulses includes only two pulses.
 7. The method ofclaim 6 wherein the second sequence of pulses includes only two pulses.8. The method of claim 1 wherein generating said first sequence ofcoherent optical pulses comprises delivering a first coherent laser beamat a first predetermined wavelength and producing said first sequence ofpulses from the first coherent laser beam.
 9. The method of claim 8wherein generating said second sequence of coherent optical pulsescomprises delivering a second coherent laser beam at the firstpredetermined wavelength and producing said second sequence of pulsesfrom the second coherent laser beam.
 10. The method of claim 1 furthercomprising encoding first data onto the first sequence of pulses beforemanipulating the pulses of the first sequence of pulses.
 11. The methodof claim 10 further comprising encoding second data onto the secondsequence of pulses before manipulating the pulses of the second sequenceof pulses.
 12. The method of claim 1 wherein introducing both the firstmanipulated sequence of pulses and the second manipulated sequence ofpulses into said optical fiber comprises passing both the firstmanipulated sequence of pulses and the second manipulated sequence ofpulses through a wavelength multiplexer.
 13. The method of claim 1wherein generating said first sequence of optical pulses comprises:supplying a first continuous single coherent wave laser beam at a firstpreselected wavelength; and chopping the first continuous wave laserbeam to produce the first sequence of optical pulses.
 14. The method ofclaim 13 wherein generating said second sequence of optical pulsescomprises: supplying a second continuous single coherent wave laser beamat a second preselected wavelength; and chopping the second continuouswave laser beam to produce the second sequence of optical pulses. 15.The method of claim 14 wherein the first and second preselectedwavelengths are the same.
 16. The method of claim 1 wherein generatingsaid first sequence of optical pulses comprises: supplying a firstsingle coherent optical pulse at a first preselected wavelength; andproducing the first sequence of optical pulses from the first singlecoherent optical pulse.
 17. The method of claim 16 wherein generatingsaid second sequence of optical pulses comprises: supplying a secondsingle coherent optical pulse at a second preselected wavelength; andproducing the second sequence of optical pulses from the second singlecoherent optical pulse.
 18. The method of claim 17 wherein the first andsecond preselected wavelengths are the same.
 19. An apparatus forsending optical signals down an optical fiber characterized bydispersion and a non-linear regime of operation, said apparatuscomprising: a first optical signal source which during operationgenerates a first sequence of coherent optical pulses each of which hasan associated energy, wherein the pulses in the first sequence ofoptical pulses are sufficiently close in spacing so that upon travelinga predetermined length down the optical fiber, the pulses of the firstsequence of pulses will overlap and interfere to form a firstinterference pattern with a first central lobe having a characteristicwavelength, and wherein the associated energy of at least one of thepulses of the first sequence of pulses is within the non-linear regimeof the optical fiber; a frequency shifter which during operationmanipulates the pulses of the first sequence of optical pulses togenerate a first manipulated sequence of pulses; a second optical signalsource which during operation generates a second sequence of coherentoptical pulses each of which has an associated energy, wherein thepulses in the second sequence of optical pulses are sufficiently closein spacing so that upon traveling a predetermined length down theoptical fiber, the pulses of the second sequence of pulses will overlapand interfere to form a second interference pattern with a secondcentral lobe having a characteristic wavelength, and wherein theassociated energy of at least one of the pulses of the second sequenceof pulses is within the non-linear regime of the optical fiber; a secondfrequency shifter which during operation manipulates the pulses of thesecond sequence of optical pulses to generate a second manipulatedsequence of pulses; and a wavelength division multiplexer system whichduring operation receives both the first manipulated sequence of pulsesand the second manipulated sequence of pulses and introduces them intosaid optical fiber, wherein the first and second frequency shiftersshift the characteristic wavelengths of the first and second centrallobes, respectively, to first and second transmission signalwavelengths.